The present invention relates to photoplotters wherein a photosensitive media is exposed in accordance with a desired graphic design.
Graphic recorders are known to the prior art. Among those that have been used are the well known pen plotters wherein a media and one or more pens are moved relative to each other to produce the desired graphics, often under the control of a numerical controller or computer. However, many applications require a control of line width beyond that easily attained with a pen plotter, partly because of difficulties in controlling the widths of the plotted lines due to ink flow problems.
The desired use may also place restrictions on the graphic recorder. For example, many manufacturing applications require a transparency which can be used as a mask, such as that used in the etching of printed circuit boards. Graphic production on another media and transfer to a transparency requires a photographic process, which may, itself, contribute to inaccuracies in the mask. Line width variations may be further compounded when there is a significant size difference between the plot, as produced, and the plot, as applied.
Because of the above-noted difficulties, photographic plotters or photoplotters have come into use. However, photoplotters tend to be very expensive, massive and slow. In a typical photoplotter, a radiant energy pattern is generated on a photosensitive media, such as a photographic film, with the media and pattern being moved relative to each other in accordance with the desired exposure trace on the media. For example, a shaped light beam may be directed at a film from a stationary light source with the film being moved relative to the pattern formed by the light beam on the film. Conventional designs employ two platforms for film support and movement, each movable in a direction orthogonal to the movement direction of the other and with one platform supported for movement with the other. With the film size requirements of modern applications, such platforms are large and heavy. Accordingly, the systems that move the platforms are necessarily massive, and often slow. A separate moving system for each platform is commonly employed, typically having a driven lead screw fastened each platform.
One common lead screw/platform drive mechanism employs a high-precision lead screw which is turned by means of a stepper motor. Maintaining the position of the platforms, and thus the film, to a high accuracy (less than 1 mil, for example) requires the use of expensive components and, again, massive supporting structures. Other approaches suggest that the film be held stationary while the device that generates the exposing light beam is moved relative to the film. While this appears to reduce the problems discussed above, those problems are reduced only in degree and continue as complicating factors.